1. Field of the Invention
The present invention relates to an image forming apparatus using an electrostatic method or electrophotographic printing method, and a control method of the image forming apparatus.
2. Description of the Related Art
Related arts for the present invention will be described by classifying them into a transfer stabilization technique and a color stabilization technique.
<Transfer Stabilization Technique>
There has conventionally been known an image forming apparatus having a plurality of image forming units.
Each image forming unit forms an electrostatic latent image on an image carrier such as a photosensitive drum by irradiating the image carrier with a modulated laser beam or light from a light emitting element such as an LED in accordance with image information. The electrostatic latent image is developed as a toner image by a developing means which stores a developer (toner). The toner image is transferred onto a transfer medium (print medium) conveyed by a transfer medium conveyor or onto an intermediate transfer member.
There is proposed an image forming apparatus which forms a color image by the following method. More specifically, a plurality of image forming units form toner images in different colors. While the transfer medium conveyor sequentially conveys a transfer medium to predetermined positions of the image forming units, the toner images in the respective colors are multiply transferred onto the transfer medium. There is also proposed an image forming apparatus which forms a color image by the following method (an intermediate transfer method). More specifically, toner images in respective colors are multiply transferred onto an intermediate transfer member, and then a toner image multiply transferred on the intermediate transfer member is transferred at once onto a transfer medium. The intermediate transfer member is an endless intermediate transfer belt which is looped between a driving roller for transferring a driving force and at least one driven roller and whose surface is moved by the driving force.
In an image forming apparatus of this type, the primary transfer current needs to be set optimally in order to increase the transfer latitude (transfer efficiency) from a photosensitive drum (to be referred to as a “drum” hereinafter) serving as an image carrier to an intermediate transfer belt. However, a small primary transfer current tends to generate a transfer failure, while a large primary transfer current tends to cause retransfer. It is difficult to optimally set the primary transfer current.
From this, the primary transfer latitude is increased by setting a peripheral speed difference between the photosensitive drum and the intermediate transfer belt. There is proposed a technique of achieving an increase in primary transfer latitude and stabilization when primarily transferring a toner image from the photosensitive drum to the intermediate transfer belt. More specifically, a toner image on the photosensitive drum is transferred to the intermediate transfer belt using a shearing force large enough to scrape the toner image away from the photosensitive drum by utilizing the peripheral speed difference. This technique prevents generation of density nonuniformity of an image and a disconnected stroke line or character image, which arise from a decrease in primary transfer latitude. In particular, the transfer latitude can increase without omitting the center part of a fine line in a secondary color. However, a frictional force always exists between the photosensitive drum and the intermediate transfer belt owing to the peripheral speed difference. The coefficient of friction changes between a case where toner exists between the photosensitive drum and the intermediate transfer belt and a case where no toner exists between them, thereby changing the rotational speed of the photosensitive drum. As a result, when forming an electrostatic latent image on the photosensitive drum, exposure fluctuates, therefore, generating a undesirable line-like image.
This phenomenon occurs even in an image forming apparatus in which a plurality of developing units are arranged for one image carrier, and toner images in a plurality of colors are sequentially formed on the image carrier and superimposed on the intermediate transfer member, thereby forming a color image. This phenomenon also occurs in a system which directly transfers a toner image from the photosensitive drum onto a transfer medium conveyed by the transfer medium conveyor. In this application, the transfer medium conveyor and intermediate transfer member will be referred to as a transfer moving means at once.
To prevent this phenomenon, the following image forming apparatus is proposed (e.g., Japanese Patent Laid-Open No. 2004-118076, hereinafter referred as JPA 2004-118076). More specifically, a peripheral speed difference is set between the rotational speed of the image forming units and that of the transfer moving means such as the intermediate transfer member or transfer medium conveyor on which a toner image is transferred. Additionally, a dispersed dots image (also referred as a dot pattern) is formed on the transfer moving means by dispersing dot developer images (dot toner images) each formed by a predetermined small dot on a normal toner image, that is, normal image.
In this way, there can be provided an image forming apparatus capable of printing a high-quality image by performing a more stable image forming, even in the arrangement in which the peripheral speed difference is set between the rotational speeds of the photosensitive drum and transfer moving means.
For example, even in an image forming apparatus in which no peripheral speed difference is set between the image carrier and the transfer moving means, an unintended peripheral speed difference is generated owing to decentering of the driving roller or the like, thereby causing color misregistration, as described in Japanese Patent Laid-Open No. 11-52758. To prevent this, similar to JPA 2004-118076, a dot pattern is formed on the transfer moving means by dispersing dot toner images each formed by a predetermined small dot on a normal image. This allows printing a high-quality image by performing a more stable image forming.
<Color Stabilization Technique>
On the other hand, these days, demand is growing for direct imaging printers using no plate. Many companies adopt direct imaging printers in consideration of shortening time till finishing printing, respective customer services, and environmental issues, that is, production and disposal in large volume. Of direct imaging printers, inkjet printers and electrophotographic printers are increasing their market shares. Because, the inkjet printer is advantageous in cost and suitable for photo printing. The electrophotographic printer has high productivity and can provide almost the same printed products as those by offset printing. In this situation, color stabilization is most important to substitute these printers for conventional offset printing and photographs.
To ensure color stabilization, stabilization control is executed in a printer. More specifically, there is known a technique (referred as a pre-fixing toner density control) of sensing, by a density sensor, a patch pattern image for detecting the toner density formed on a photosensitive drum, and feeding back the detected toner density to a toner density controller in a developing unit, thereby properly controlling the toner density (Japanese Patent Laid-Open No. 1-309082).
Generally, a toner patch can be easily formed and erased, but can provide only information on toner density before fixing toner on a transfer medium. Hence, by this toner patch based control, the influence of the fixing process and subsequent processes cannot be reflected on the detected toner density.
From this, it is proposed to read a printed image by the so-called reader of a copying machine assembled into the apparatus main body, and control the image (referred as a post-fixing patch reading by reader) (Japanese Patent Laid-Open No. 63-185279).
However, this technique is poor in operability because the user must carry a transfer medium on which an output image is formed to the reader. In many cases, the user does not periodically print a patch image and control toner density because it is troublesome. As a technique which removes the burden on the user, there is disclosed a technique of sensing a fixed output image by arranging a density sensor midway along the post-fixing conveyance path (Japanese Patent Laid-Open No. 10-193689).
There is also disclosed a technique of adjusting achromatic color balance (gray balance) sensitive to the human sense of vision upon detecting a color image (referred as a post-fixing patch color image sensing by a sensor) (Japanese Patent Laid-Open No. 2002-344759).
Thus, color stabilization is one of most important issues even in a direct imaging type image forming apparatus. Color stabilization control using a sensor arranged after the image forming and fixing receives attention.
However, if a dispersed dots image for transfer stabilization is formed on the entire surface of a sheet when forming a patch image for color stabilization control, the measurement value is influenced by toner increased by the dispersed dots image according to the method of measuring the toner density of the patch image before fixing toner.
According to the method of measuring the toner density of a toner patch after fixing toner, a dispersed dots image is directly transferred onto a sheet. When the reader or color sensor senses the patch image, therefore, the sensed data is affected by the dispersed dots image.
For example, a yellow dispersed dots image is formed over even a cyan halftone patch image. When the reader or color sensor senses the patch image, the sensed data contains the yellow toner component. If no dispersed dots image is formed when forming a patch image in order to avoid this influence, a undesirable line-like image is formed on the patch image due to a change of the coefficient of friction, and the sensed density value fluctuates as shown in FIG. 22, thereby failing accurate measuring.